Page 916 - The Veterinary Care of the Horse

• demonstrating aggressive behaviour

• interest in mares
VetBooks.ir • attempting to cover mares in season

•
stallion-like appearance, e.g. a well-developed crest.

Although retained testicles produce normal amounts of male hormone, they are incapable of
producing sperm due to the body heat of the horse. Thus a bilateral rig is likely to be infertile,

whilst the unilateral rig is usually fertile since the descended testicle will be functioning
normally. There are stallions that have bred successfully as unilateral rigs, but this is
controversial. Most vets advise against it because cryptorchidism is an inherited trait and so

by breeding from them, there is a considerable chance of producing more defective progeny.
One USA study that looked at more than 5000 cryptorchid horses noted an increased

incidence in some breeds. The highest incidence was found in Percherons, American
Saddlebred horses, Quarter horses, and ponies and crossbred horses. However, in the

Thoroughbred, Standardbred, Morgan, Tennessee Walking Horse and Arabian, the incidence
was less than would have been expected statistically. This may have been a reflection of

breeding policy.

Diagnosis

If the castration history of a particular animal is unknown and its behaviour gives cause for

concern, blood tests can be used to determine whether a testicle is present.
• If the horse is over 3 years, the serum levels of oestrone sulphate are much higher in a

horse with testicular tissue
• In younger horses, the testosterone levels are measured before and after an intravenous

injection of human chorionic gonadotrophin (hCG). The testosterone level will rise 2-3

times in a horse with testicular tissue and remain unaffected in geldings
• Anti-Müllerian hormone (AMH) is produced by descended and retained testicles. This

blood test is useful for identifying cryptorchid horses that have had one testicle removed.
It improves the accuracy of diagnosis when used in combination with either of the above

tests. The serum AMH levels are higher in cryptorchid stallions compared with intact
stallions; the levels are not detectable in geldings.

• Ultrasonography of the inguinal region and caudal abdomen can be helpful in the
diagnosis of cryptorchidism and location of the retained testicle.

WHERE IS THE MISSING TESTICLE?